Slow query when using datediff function in Oracle

As my title, I have the following code:

 SELECT
        *
    FROM
        de.Department
    WHERE
        de.flag = 1
        AND   de.DepartmentNum IN (10,4)
        AND   de.status IN (0,-1,100)
        AND   datediff('dd',de.datequit,'30-SEP-19') > 9

The datediff function make my query run very slow(16s for 11 records), and cost also very high(~43k).

Here is my datediff function code

create or replace FUNCTION       DATEDIFF 
(
  P_TYPE_DATE IN VARCHAR2 
, P_START_DATE IN TIMESTAMP 
, P_END_DATE IN TIMESTAMP 
) RETURN NUMBER AS 
  v_Result NUMBER := -1;
BEGIN

  IF P_TYPE_DATE IS NOT NULL AND P_START_DATE IS NOT NULL AND P_END_DATE IS NOT NULL THEN
    CASE UPPER(P_TYPE_DATE) 
        WHEN 'DD' THEN RETURN ROUND(TRUNC(P_END_DATE,'DD') - TRUNC(P_START_DATE,'DD'),0);
        WHEN 'HH' THEN RETURN ROUND((TRUNC(P_END_DATE,'HH') - TRUNC(P_START_DATE,'HH')) * 24,0);
        WHEN 'MI' THEN RETURN ROUND((TRUNC(P_END_DATE,'MI') - TRUNC(P_START_DATE,'MI')) * 24 * 60,0);
        WHEN 'SS' THEN RETURN ROUND((TRUNC(P_END_DATE,'MI') - TRUNC(P_START_DATE,'MI')) * 24 * 60 * 60 + extract(second from (P_END_DATE - P_START_DATE)),0);
        ELSE RETURN NULL;
    END CASE;
  END IF;
  RETURN NULL;
  EXCEPTION
    WHEN OTHERS THEN
       raise_application_error(-20001,'An error was encountered - '||SQLCODE||' -ERROR- '||SQLERRM);
END DATEDIFF;

I used SELECT * because I want to get almost column in Department table, so it no more change if I SELECT some columns which I need.

Can I re-write to improve performance and cost?

Mayny thanks!

Answer

I created a function named datediff as a datediff function in SQL, sir

Don’t use custom functions as they prevent Oracle from using an index on the column; instead just compare the column to the static values:

SELECT *
FROM   Department
WHERE  flag = 1
AND    DepartmentNum IN (10,4)
AND    status IN (0,-1,100)
AND    datequit > DATE '2019-09-30' + INTERVAL '9' DAY

or

AND    datequit > DATE '2019-09-30' + NUMTODSINTERVAL( 9, 'DAY' )

or

AND    datequit > DATE '2019-09-30' + 9

Here is my datediff function code

...
WHEN 'DD' THEN RETURN ROUND(TRUNC(P_END_DATE,'DD') - TRUNC(P_START_DATE,'DD'),0);
...

If you want to do an equivalent comparison to using TRUNC to ignore the time components then change from using greater-than comparison to using greater-than-or-equal-to and add one time unit (day in your example) to the expected difference. For example:

SELECT *
FROM   Department
WHERE  flag = 1
AND    DepartmentNum IN (10,4)
AND    status IN (0,-1,100)
AND    datequit >= DATE '2019-09-30' + INTERVAL '10' DAY